CSI feedback overhead reduction for FD-MIMO

ABSTRACT

Mechanisms for reduction of channel state information (CSI) feedback overhead are disclosed for full dimensional multiple input, multiple output (FD-MIMO) systems with large dimension antenna ports. In one aspect, rank-dependent CSI antenna port measurements are used in order to limit the number of antenna ports for high rank CSI reporting. Another aspect allows a user equipment (UE) to select subband feedback for aperiodic CSI porting on an uplink shared channel when the UE is to report subband quality and precoding indicators. Another aspect provides for on-demand CSI feedback that dynamically configures CSI feedback parameters. To reduce the signaling overhead, the multiple parameter sets may be pre-configured with different values for dynamic reporting parameters.

BACKGROUND

Field

Aspects of the present disclosure relate generally to wirelesscommunication systems, and more particularly, to channel stateinformation (CSI) feedback overhead reduction for full-dimensionalmultiple-input, multiple-output (MIMO) systems.

Background

Wireless communication networks are widely deployed to provide variouscommunication services such as voice, video, packet data, messaging,broadcast, etc. These wireless networks may be multiple-access networkscapable of supporting multiple users by sharing the available networkresources. Examples of such multiple-access networks include CodeDivision Multiple Access (CDMA) networks, Time Division Multiple Access(TDMA) networks, Frequency Division Multiple Access (FDMA) networks,Orthogonal FDMA (OFDMA) networks, and Single-Carrier FDMA (SC-FDMA)networks.

A wireless communication network may include a number of base stationsor node Bs that can support communication for a number of userequipments (UEs). A UE may communicate with a base station via downlinkand uplink. The downlink (or forward link) refers to the communicationlink from the base station to the UE, and the uplink (or reverse link)refers to the communication link from the UE to the base station.

A base station may transmit data and control information on the downlinkto a UE and/or may receive data and control information on the uplinkfrom the UE. On the downlink, a transmission from the base station mayencounter interference due to transmissions from neighbor base stationsor from other wireless radio frequency (RF) transmitters. On the uplink,a transmission from the UE may encounter interference from uplinktransmissions of other UEs communicating with the neighbor base stationsor from other wireless RF transmitters. This interference may degradeperformance on both the downlink and uplink.

As the demand for mobile broadband access continues to increase, thepossibilities of interference and congested networks grows with more UEsaccessing the long-range wireless communication networks and moreshort-range wireless systems being deployed in communities. Research anddevelopment continue to advance communication technologies not only tomeet the growing demand for mobile broadband access, but to advance andenhance the user experience with mobile communications.

SUMMARY

In one aspect of the disclosure, a method of wireless communicationincludes receiving, at a UE, an indication to report channel quality andprecoding feedback for a plurality of channel state informationreference signal (CSI-RS) ports in a full dimensional multiple input,multiple output (FD-MIMO) downlink transmission, transmitting, by theUE, a first channel quality and precoding feedback for each of theplurality of CSI-RS ports, wherein the first channel quality andprecoding feedback includes a low rank indicator, selecting, by the UE,a second set of CSI-RS ports of the plurality of CSI-RS ports for asecond channel quality and precoding feedback, wherein the selecting thesecond set of CSI-RS ports is based on a rank indicator associated withthe second channel quality and precoding feedback, and transmitting, bythe UE, a second channel quality and precoding feedback for each of asecond set of CSI-RS ports of the plurality of CSI-RS ports.

In one aspect of the disclosure, a method of wireless communicationincludes receiving, at a UE, an indication to report the subband channelquality and precoding feedback for a plurality of subbands over acarrier bandwidth, selecting, by the UE, a subset of the plurality ofsubbands for reporting the subband channel quality and precodingfeedback, transmitting, by the UE, the subband channel quality andprecoding feedback for each of the subbands in the subset, andtransmitting, by the UE, a subband selection indicator to indicate thelocation of the subset of the plurality of subbands.

In one aspect of the disclosure, a method of wireless communicationincludes receiving, at a UE, an indication to report aperiodic CSIfeedback on an uplink shared channel in a FD-MIMO downlink transmission,wherein the indication includes a feedback parameter code, looking up,at the UE, a list of feedback parameters associated with the feedbackparameter code, and generating, by the UE, an aperiodic CSI feedbackreport using each of the list of feedback parameters.

In one aspect of the disclosure, an apparatus configured for wirelesscommunication includes means for receiving, at a UE, an indication toreport channel quality and precoding feedback for a plurality of CSI-RSports in a FD-MIMO downlink transmission, means for transmitting, by theUE, a first channel quality and precoding feedback for each of theplurality of CSI-RS ports, wherein the first channel quality andprecoding feedback includes a low rank indicator, means for selecting,by the UE, a second set of CSI-RS ports of the plurality of CSI-RS portsfor a second channel quality and precoding feedback, wherein the meansfor selecting the second set of CSI-RS ports is based on a rankindicator associated with the second channel quality and precodingfeedback, and means for transmitting, by the UE, a second channelquality and precoding feedback for each of a second set of CSI-RS portsof the plurality of CSI-RS ports.

The foregoing has outlined rather broadly the features and technicaladvantages of the present application in order that the detaileddescription that follows may be better understood. Additional featuresand advantages will be described hereinafter which form the subject ofthe claims. It should be appreciated by those skilled in the art thatthe conception and specific aspect disclosed may be readily utilized asa basis for modifying or designing other structures for carrying out thesame purposes of the present application. It should also be realized bythose skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the present application and theappended claims. The novel features which are believed to becharacteristic of aspects, both as to its organization and method ofoperation, together with further objects and advantages will be betterunderstood from the following description when considered in connectionwith the accompanying figures. It is to be expressly understood,however, that each of the figures is provided for the purpose ofillustration and description only and is not intended as a definition ofthe limits of the present claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of atelecommunications system.

FIG. 2 is a block diagram illustrating an example of a down link framestructure in a telecommunications system.

FIG. 3 is a block diagram illustrating a design of a base station and aUE configured according to one aspect of the present disclosure.

FIG. 4 is block diagram of an exemplary two-dimensional active antennaarray.

FIG. 5 is a block diagram illustrating a two CSI process configurationeach with one dimensional CSI-RS ports for the dimensional CSI feedback.

FIG. 6 is a block diagram illustrating a base station transmittingprecoded CSI-RS.

FIG. 7 is a block diagram illustrating example blocks executed toimplement one aspect of the present disclosure.

FIG. 8 is a block diagram illustrating a base station and UE configuredto reduce CSI feedback in FD-MIMO transmissions according to aspects ofthe present disclosure.

FIG. 9 is a block diagram illustrating example blocks executed toimplement one aspect of the present disclosure.

FIGS. 10A and 10B are block diagrams illustrating example subbandselection according to aspects of the present disclosure.

FIG. 11 is a block diagram illustrating example blocks executed toimplement one aspect of the present disclosure.

FIG. 12 is a block diagram illustrating a UE configured according to oneaspect of the present disclosure.

DETAILED DESCRIPTION

The detailed description set forth below, in connection with theappended drawings, is intended as a description of variousconfigurations and is not intended to represent the only configurationsin which the concepts described herein may be practiced. The detaileddescription includes specific details for the purpose of providing athorough understanding of the various concepts. However, it will beapparent to those skilled in the art that these concepts may bepracticed without these specific details. In some instances, well-knownstructures and components are shown in block diagram form in order toavoid obscuring such concepts.

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), CDMA2000, etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. CDMA2000 coversIS-2000, IS-95 and IS-856 standards. A TDMA network may implement aradio technology such as Global System for Mobile Communications (GSM).An OFDMA network may implement a radio technology such as Evolved UTRA(E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, Flash-OFDMA, etc. UTRA and E-UTRA are part ofUniversal Mobile Telecommunication System (UMTS). 3GPP Long TermEvolution (LTE) and LTE-Advanced (LTE-A) are new releases of UMTS thatuse E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described indocuments from an organization named “3rd Generation PartnershipProject” (3GPP). CDMA2000 and UMB are described in documents from anorganization named “3rd Generation Partnership Project 2” (3GPP2). Thetechniques described herein may be used for the wireless networks andradio technologies mentioned above as well as other wireless networksand radio technologies. For clarity, certain aspects of the techniquesare described below for LTE, and LTE terminology is used in much of thedescription below.

FIG. 1 shows a wireless communication network 100, which may be an LTEnetwork. The wireless network 100 may include a number of eNBs 110 andother network entities. An eNB may be a station that communicates withthe UEs and may also be referred to as a base station, a Node B, anaccess point, or other term. Each eNB 110 a, 110 b, 110 c may providecommunication coverage for a particular geographic area. In 3GPP, theterm “cell” can refer to a coverage area of an eNB and/or an eNBsubsystem serving this coverage area, depending on the context in whichthe term is used.

An eNB may provide communication coverage for a macro cell, a pico cell,a femto cell, and/or other types of cell. A macro cell may cover arelatively large geographic area (e.g., several kilometers in radius)and may allow unrestricted access by UEs with service subscription. Apico cell may cover a relatively small geographic area and may allowunrestricted access by UEs with service subscription. A femto cell maycover a relatively small geographic area (e.g., a home) and may allowrestricted access by UEs having association with the femto cell (e.g.,UEs in a Closed Subscriber Group (CSG), UEs for users in the home,etc.). An eNB for a macro cell may be referred to as a macro eNB. An eNBfor a pico cell may be referred to as a pico eNB. An eNB for a femtocell may be referred to as a femto eNB or a home eNB (HeNB). In theexample shown in FIG. 1, the eNBs 110 a, 110 b and 110 c may be macroeNBs for the macro cells 102 a, 102 b and 102 c, respectively. The eNB110 x may be a pico eNB for a pico cell 102 x, serving a UE 120 x. TheeNBs 110 y and 110 z may be femto eNBs for the femto cells 102 y and 102z, respectively. An eNB may support one or multiple (e.g., three) cells.

The wireless network 100 may also include relay stations 110 r. A relaystation is a station that receives a transmission of data and/or otherinformation from an upstream station (e.g., an eNB or a UE) and sends atransmission of the data and/or other information to a downstreamstation (e.g., a UE or an eNB). A relay station may also be a UE thatrelays transmissions for other UEs. In the example shown in FIG. 1, arelay station 110 r may communicate with the eNB 110 a and a UE 120 r inorder to facilitate communication between the eNB 110 a and the UE 120r. A relay station may also be referred to as a relay eNB, a relay, etc.

The wireless network 100 may be a heterogeneous network that includeseNBs of different types, e.g., macro eNBs, pico eNBs, femto eNBs,relays, etc. These different types of eNBs may have different transmitpower levels, different coverage areas, and different impact oninterference in the wireless network 100. For example, macro eNBs mayhave a high transmit power level (e.g., 20 Watts) whereas pico eNBs,femto eNBs and relays may have a lower transmit power level (e.g., 1Watt).

The wireless network 100 may support synchronous or asynchronousoperation. For synchronous operation, the eNBs may have similar frametiming, and transmissions from different eNBs may be approximatelyaligned in time. For asynchronous operation, the eNBs may have differentframe timing, and transmissions from different eNBs may not be alignedin time. The techniques described herein may be used for bothsynchronous and asynchronous operation.

A network controller 130 may couple to a set of eNBs and providecoordination and control for these eNBs. The network controller 130 maycommunicate with the eNBs 110 via a backhaul. The eNBs 110 may alsocommunicate with one another, e.g., directly or indirectly via wirelessor wireline backhaul.

The UEs 120 may be dispersed throughout the wireless network 100, andeach UE may be stationary or mobile. A UE may also be referred to as aterminal, a mobile station, a subscriber unit, a station, etc. A UE maybe a cellular phone, a personal digital assistant (PDA), a wirelessmodem, a wireless communication device, a handheld device, a laptopcomputer, a cordless phone, a smart phone, a tablet, a wireless localloop (WLL) station, or other mobile entities. A UE may be able tocommunicate with macro eNBs, pico eNBs, femto eNBs, relays, or othernetwork entities. In FIG. 1, a solid line with double arrows indicatesdesired transmissions between a UE and a serving eNB, which is an eNBdesignated to serve the UE on the downlink and/or uplink. A dashed linewith double arrows indicates interfering transmissions between a UE andan eNB.

LTE utilizes orthogonal frequency division multiplexing (OFDM) on thedownlink and single-carrier frequency division multiplexing (SC-FDM) onthe uplink. OFDM and SC-FDM partition the system bandwidth into multiple(K) orthogonal subcarriers, which are also commonly referred to astones, bins, etc. Each subcarrier may be modulated with data. Ingeneral, modulation symbols are sent in the frequency domain with OFDMand in the time domain with SC-FDM. The spacing between adjacentsubcarriers may be fixed, and the total number of subcarriers (K) may bedependent on the system bandwidth. For example, K may be equal to 128,256, 512, 1024 or 2048 for system bandwidth of 1.25, 2.5, 5, 10 or 20megahertz (MHz), respectively. The system bandwidth may also bepartitioned into subbands. For example, a subband may cover 1.08 MHz,and there may be 1, 2, 4, 8 or 16 subbands for system bandwidth of 1.25,2.5, 5, 10 or 20 MHz, respectively.

FIG. 2 shows a down link frame structure used in LTE. The transmissiontimeline for the downlink may be partitioned into units of radio frames.Each radio frame may have a predetermined duration (e.g., 10milliseconds (ms)) and may be partitioned into 10 subframes with indicesof 0 through 9. Each subframe may include two slots. Each radio framemay thus include 20 slots with indices of 0 through 19. Each slot mayinclude L symbol periods, e.g., 7 symbol periods for a normal cyclicprefix (CP), as shown in FIG. 2, or 6 symbol periods for an extendedcyclic prefix. The normal CP and extended CP may be referred to hereinas different CP types. The 2L symbol periods in each subframe may beassigned indices of 0 through 2L−1. The available time frequencyresources may be partitioned into resource blocks. Each resource blockmay cover N subcarriers (e.g., 12 subcarriers) in one slot.

In LTE, an eNB may send a primary synchronization signal (PSS) and asecondary synchronization signal (SSS) for each cell in the eNB. Theprimary and secondary synchronization signals may be sent in symbolperiods 6 and 5, respectively, in each of subframes 0 and 5 of eachradio frame with the normal cyclic prefix, as shown in FIG. 2. Thesynchronization signals may be used by UEs for cell detection andacquisition. The eNB may send a Physical Broadcast Channel (PBCH) insymbol periods 0 to 3 in slot 1 of subframe 0. The PBCH may carrycertain system information.

The eNB may send a Physical Control Format Indicator Channel (PCFICH) inonly a portion of the first symbol period of each subframe, althoughdepicted in the entire first symbol period in FIG. 2. The PCFICH mayconvey the number of symbol periods (M) used for control channels, whereM may be equal to 1, 2 or 3 and may change from subframe to subframe. Mmay also be equal to 4 for a small system bandwidth, e.g., with lessthan 10 resource blocks. In the example shown in FIG. 2, M=3. The eNBmay send a Physical HARQ Indicator Channel (PHICH) and a PhysicalDownlink Control Channel (PDCCH) in the first M symbol periods of eachsubframe (M=3 in FIG. 2). The PHICH may carry information to supporthybrid automatic retransmission (HARQ). The PDCCH may carry informationon resource allocation for UEs and control information for downlinkchannels. Although not shown in the first symbol period in FIG. 2, it isunderstood that the PDCCH and PHICH are also included in the firstsymbol period. Similarly, the PHICH and PDCCH are also both in thesecond and third symbol periods, although not shown that way in FIG. 2.The eNB may send a Physical Downlink Shared Channel (PDSCH) in theremaining symbol periods of each subframe. The PDSCH may carry data forUEs scheduled for data transmission on the downlink. The various signalsand channels in LTE are described in 3GPP TS 36.211, entitled “EvolvedUniversal Terrestrial Radio Access (E-UTRA); Physical Channels andModulation,” which is publicly available.

The eNB may send the PSS, SSS and PBCH in the center 1.08 MHz of thesystem bandwidth used by the eNB. The eNB may send the PCFICH and PHICHacross the entire system bandwidth in each symbol period in which thesechannels are sent. The eNB may send the PDCCH to groups of UEs incertain portions of the system bandwidth. The eNB may send the PDSCH tospecific UEs in specific portions of the system bandwidth. The eNB maysend the PSS, SSS, PBCH, PCFICH and PHICH in a broadcast manner to allUEs, may send the PDCCH in a unicast manner to specific UEs, and mayalso send the PDSCH in a unicast manner to specific UEs.

A number of resource elements may be available in each symbol period.Each resource element may cover one subcarrier in one symbol period andmay be used to send one modulation symbol, which may be a real orcomplex value. Resource elements not used for a reference signal in eachsymbol period may be arranged into resource element groups (REGs). EachREG may include four resource elements in one symbol period. The PCFICHmay occupy four REGs, which may be spaced approximately equally acrossfrequency, in symbol period 0. The PHICH may occupy three REGs, whichmay be spread across frequency, in one or more configurable symbolperiods. For example, the three REGs for the PHICH may all belong insymbol period 0 or may be spread in symbol periods 0, 1 and 2. The PDCCHmay occupy 9, 18, 32 or 64 REGs, which may be selected from theavailable REGs, in the first M symbol periods. Only certain combinationsof REGs may be allowed for the PDCCH.

A UE may know the specific REGs used for the PHICH and the PCFICH. TheUE may search different combinations of REGs for the PDCCH. The numberof combinations to search is typically less than the number of allowedcombinations for the PDCCH. An eNB may send the PDCCH to the UE in anyof the combinations that the UE will search.

A UE may be within the coverage of multiple eNBs. One of these eNBs maybe selected to serve the UE. The serving eNB may be selected based onvarious criteria such as received power, path loss, signal-to-noiseratio (SNR), etc.

FIG. 3 shows a block diagram of a design of a base station/eNB 110 and aUE 120, which may be one of the base stations/eNBs and one of the UEs inFIG. 1. For a restricted association scenario, the base station 110 maybe the macro eNB 110 c in FIG. 1, and the UE 120 may be the UE 120 y.The base station 110 may also be a base station of some other type. Thebase station 110 may be equipped with antennas 334 a through 334 t, andthe UE 120 may be equipped with antennas 352 a through 352 r.

At the base station 110, a transmit processor 320 may receive data froma data source 312 and control information from a controller/processor340. The control information may be for the PBCH, PCFICH, PHICH, PDCCH,etc. The data may be for the PDSCH, etc. The processor 320 may process(e.g., encode and symbol map) the data and control information to obtaindata symbols and control symbols, respectively. The processor 320 mayalso generate reference symbols, e.g., for the PSS, SSS, andcell-specific reference signal. A transmit (TX) multiple-inputmultiple-output (MIMO) processor 330 may perform spatial processing(e.g., precoding) on the data symbols, the control symbols, and/or thereference symbols, if applicable, and may provide output symbol streamsto the modulators (MODs) 332 a through 332 t. Each modulator 332 mayprocess a respective output symbol stream (e.g., for OFDM, etc.) toobtain an output sample stream. Each modulator 332 may further process(e.g., convert to analog, amplify, filter, and upconvert) the outputsample stream to obtain a downlink signal. Downlink signals frommodulators 332 a through 332 t may be transmitted via the antennas 334 athrough 334 t, respectively.

At the UE 120, the antennas 352 a through 352 r may receive the downlinksignals from the base station 110 and may provide received signals tothe demodulators (DEMODs) 354 a through 354 r, respectively. Eachdemodulator 354 may condition (e.g., filter, amplify, downconvert, anddigitize) a respective received signal to obtain input samples. Eachdemodulator 354 may further process the input samples (e.g., for OFDM,etc.) to obtain received symbols. A MIMO detector 356 may obtainreceived symbols from all the demodulators 354 a through 354 r, performMIMO detection on the received symbols if applicable, and providedetected symbols. A receive processor 358 may process (e.g., demodulate,deinterleave, and decode) the detected symbols, provide decoded data forthe UE 120 to a data sink 360, and provide decoded control informationto a controller/processor 380.

On the uplink, at the UE 120, a transmit processor 364 may receive andprocess data (e.g., for the PUSCH) from a data source 362 and controlinformation (e.g., for the PUCCH) from the controller/processor 380. Theprocessor 364 may also generate reference symbols for a referencesignal. The symbols from the transmit processor 364 may be precoded by aTX MIMO processor 366 if applicable, further processed by the modulators354 a through 354 r (e.g., for SC-FDM, etc.), and transmitted to thebase station 110. At the base station 110, the uplink signals from theUE 120 may be received by the antennas 334, processed by thedemodulators 332, detected by a MIMO detector 336 if applicable, andfurther processed by a receive processor 338 to obtain decoded data andcontrol information sent by the UE 120. The processor 338 may providethe decoded data to a data sink 339 and the decoded control informationto the controller/processor 340.

The controllers/processors 340 and 380 may direct the operation at thebase station 110 and the UE 120, respectively. The processor 340 and/orother processors and modules at the base station 110 may perform ordirect the execution of various processes for the techniques describedherein. The processor 380 and/or other processors and modules at the UE120 may also perform or direct the execution of the functional blocksillustrated in FIGS. 7, 9, and 11, and/or other processes for thetechniques described herein. The memories 342 and 382 may store data andprogram codes for the base station 110 and the UE 120, respectively. Ascheduler 344 may schedule UEs for data transmission on the downlinkand/or uplink.

In one configuration, the UE 120 for wireless communication includesmeans for detecting interference from an interfering base station duringa connection mode of the UE, means for selecting a yielded resource ofthe interfering base station, means for obtaining an error rate of aphysical downlink control channel on the yielded resource, and means,executable in response to the error rate exceeding a predeterminedlevel, for declaring a radio link failure. In one aspect, theaforementioned means may be the processor(s), the controller/processor380, the memory 382, the receive processor 358, the MIMO detector 356,the demodulators 354 a, and the antennas 352 a configured to perform thefunctions recited by the aforementioned means. In another aspect, theaforementioned means may be a module or any apparatus configured toperform the functions recited by the aforementioned means.

In order to increase system capacity, full-dimensional (FD)-MIMOtechnology has been considered, in which an eNB uses a two-dimensional(2D) active antenna array with a large number of antennas with antennaports having both horizontal and vertical axes, and has a larger numberof transceiver units. For conventional MIMO systems, beamforming hastypically implemented using only azimuth dimension, although of a 3Dmulti-path propagation. However, for FD-MIMO each transceiver unit hasits own independent amplitude and phase control. Such capabilitytogether with the 2D active antenna array allows the transmitted signalto be steered not only in the horizontal direction, as in conventionalmulti-antenna systems, but also simultaneously in both the horizontaland the vertical direction, which provides more flexibility in shapingbeam directions from an eNB to a UE. Thus, FD-MIMO technologies may takeadvantage of both azimuth and elevation beamforming, which would greatlyimprove MIMO system capacity.

FIG. 4 is a block diagram illustrating a typical 2D active antenna array40. Active antenna array 40 is a 64-transmitter, cross-polarized uniformplanar antenna array comprising four columns, in which each columnincludes eight cross-polarized vertical antenna elements. Active antennaarrays are often described according to the number of antenna columns(N), the polarization type (P), and the number of vertical elementshaving the same polarization type in one column (M). Thus, activeantenna array 40 has four columns (N=4), with eight vertical (M=8)cross-polarized antenna elements (P=2).

For a 2D array structure, in order to exploit the vertical dimension byelevation beamforming the channel state information (CSI) is needed atthe base station. The CSI, in terms of precoding matrix indicator (PMI)rank indicator (RI) and channel quality indicator (CQI), can be fed backto the base station by a mobile station based on downlink channelestimation and predefined PMI codebook(s). However, different from theconventional MIMO system, the eNB capable of FD-MIMO is typicallyequipped with a large scale antenna system and, thus, the acquisition offull array CSI from the UE is quite challenging due to the complexity ofchannel estimation and both excessive downlink CSI-RS overhead anduplink CSI feedback overhead.

Solutions for FD-MIMO CSI feedback mechanisms have been proposed forFD-MIMO with a large scale two-dimensional antenna array. For example,dimensional CSI feedback provides for a UE to be configured with two CSIprocesses each with a 1D CSI-RS port structure either on elevation orazimuth direction. FIG. 5 is a block diagram illustrating a two CSIprocesses configuration each with one dimensional CSI-RS ports for thedimensional CSI feedback. In dimensional CSI feedback, CSI processeswill be defined for both elevation CSI-RS ports 500 and azimuth CSI-RSports 501. The CSI feedback for each configured CSI process will reflectonly a one dimensional channel state information. For example, one CSIfeedback will only reflect the CSI of elevation CSI-RS ports 500. Theserving eNB (not shown) may then determine a correlation between the twoseparate CSI processes to obtain an estimated full antenna arrayprecoding. For example, the eNB may use the Kronecker product to combinetwo precoding vectors for the full antenna array precoding.

Another example CSI feedback mechanism employs a precoded CSI-RS withbeam selection. FIG. 6 is a block diagram illustrating a base station600 configured to transmit precoded CSI-RS for CSI feedback. The UEs inUE groups #1 and #2 are positioned at various elevations in relation tobase station 600. In a precoded CSI-RS with beam selection, CSI-RSvirtualization may be used to compress a large number of antenna portsinto a fewer number of precoded CSI-RS ports. The CSI-RS ports with thesame virtualization or elevation beamforming may be associated with oneCSI process. For example, the CSI-RS Resource #1 may include CSI-RSports with the same virtualization or elevation beamforming and would beassociated with a first CSI process, while CSI-RS Resource #2 and #3would also be associated with a different CSI process. A UE can beconfigured with one or multiple CSI processes for CSI feedback, eachwith different CSI-RS virtualization. In one example, UE 604 of UE group#1 would be configured for three CSI processes to provide measurementinformation on CSI-RS Resources #1, #2, and #3, respectively. Theserving eNB, base station 600, would determine the best serving CSI-RSbeam for UE 604 based on reported CSI feedback.

Several problems and challenges exist with the different currentsolutions for FD-MIMO CSI feedback. The current solutions each onlysupply a subset of the CSI information of the full dimensional channelin order to reduce the processing complexity and feedback overhead forthe UE. However, with only a portion of the CSI information for the fullchannel, the base station will not have the best data or information inorder to maximize communication performance. In order to get the bestdata to maximize performance, full dimensional CSI feedback with 2DPMIs, where CSI is measured from 2-dimensional CSI-RS ports with jointselection of azimuth and elevation PMIs would be ideal. A fulldimensional CSI-RS resource configuration would include more than 8CSI-RS ports which are on both horizontal and vertical directions. Ajoint selection of azimuth and elevation PMIs would performed by a UEbased on full channel measurement. In this type of full dimensionalfeedback, no Kronecker approximation would be needed for CSI reporting.However, such full dimensional CSI feedback uses a large amount ofuplink feedback overhead in order to deal with the large number ofantenna ports.

Currently, for periodic CSI on PUCCH, the maximum CSI payload size is 11bits. For CSI on PUSCH, the payload size can be larger, but it is stilllimited to the assigned uplink bandwidth. The CSI payload may also bedetermined based on the CSI reporting mode, which is configured throughRRC signaling. For example, PUSCH mode 3-2 provides for the UE to reportboth subband CQI and subband PMI with larger feedback overhead thanother CSI modes. For FD-MIMO, the optimization on the elevationbeamforming design may depend on multiple factors, such as channelcondition, available uplink bandwidth, single user/multiple useroperation, and the like. In an example of use with cell edge users,higher spatial resolution is used in order to maximize the signalstrength for entities at the cell edge. In other words, a UE is notrequired to provide accurate full dimensional CSI feedback at all times.Aspects of the present disclosure may provide for both the UE and theeNB to have the flexibility to control full CSI feedback granularity andaccuracy.

FIG. 7 is a block diagram illustrating example blocks executed toimplement one aspect of the present disclosure. The blocks and featuresillustrated in FIG. 7 will also be described with respect to thehardware and components illustrated in FIG. 8. FIG. 8 is a block diagramillustrating a base station 800 and UE 801 configured to reduce CSIfeedback in FD-MIMO transmissions according to aspects of the presentdisclosure. Base station 800, which may include components similar tothose detailed with regard to base station 110 in FIG. 3, includes a2D-MIMO active antenna array 800-AAA having four sets of elevation portsand eight sets of azimuth ports. At block 700, a UE, such as UE 801,receives an indication to report CQI/PMI feedback for a plurality ofCSI-RS ports in a FD-MIMO downlink transmission from base station 800using 2D-MIMO active antenna array 800-AAA.

At block 701, UE 801 may transmit a first COI/PMI feedback for each ofthe plurality of CSI-RS ports, wherein the first COI/PMI feedbackincludes a low rank indicator. Accordingly, UE 801 generates thelower-order rank CQI/PMI feedback for each select a first set of CSI-RSports of the plurality of CSI-RS ports of 2D-MIMO active antenna array800-AAA for a first CQI/PMI feedback, wherein the first CQI/PMI feedbackis associated with a low rank indicator. At block 702, UE 801 may selecta second set of CSI-RS ports of the plurality of CSI-RS ports for asecond CQI/PMI feedback. Accordingly, the described aspect provides forrank-dependent CSI measurement. In example aspects, the selection of thesecond set of CSI-RS ports is based on a rank indicator associated withthe second CQI/PMI feedback.

At block 703, UE 801 may transmit the second CQI/PMI feedback for asecond set of CSI-RS ports of 2D-MIMO active antenna array 800-AAA. Byconsidering rank-dependent limitations on the number of CSI-RS ports forCSI feedback, UE 801 effectively reduces the uplink overhead byselecting only a subset of the total number of CSI-RS ports of 2D-MIMOactive antenna array 800-AAA for high rank CSI feedback. In variousexamples of operation, rank 1 feedback may be based on a full set of theplurality of CSI-RS ports and rank 2 and other higher order rankfeedback may be based on a part of CSI-RS ports of the plurality ofCSI-RS ports. For rank 1, UE 801 would only report PMI for one layer.Thus, with the same feedback overhead, rank 1 could support a largernumber of CSI-RS ports than rank 2 or other higher order ranks.

Secondly, rank 1 feedback is typically associated with multiuseroperation. Compared with single user MIMO, the elevation beamformingdesign for multiuser MIMO may provide for higher spatial resolution andfiner precoding granularity. Thus, the rank-dependent CSI limitation ormanagement allows for a tradeoff between single user MIMO performanceand uplink feedback overhead, which may also help to reduce UEprocessing complexity.

There are several options which may be utilized by UE 801 to determinewhich CSI-RS ports of 2D-MIMO active antenna array 800-AAA to designatefor higher order single user MIMO. In one option, CSI-RS ports forhigher order rank may be considered a subset of the CSI-RS ports in2D-MIMO active antenna array 800-AAA available for rank 1. For example,the subset of higher order single user MIMO CSI-RS ports of 2D-MIMOactive antenna array 800-AAA may be configured via RRC signaling of abitmap indicator from base station 800.

In a second option, RRC signaling from base station 800 may providefixed weights to designate virtualized CSI-RS ports. The weightsreceived by UE 801 from base station 800 may be used to generate avirtualization matrix, T. For example, with a full channel matrix, H,and a configured CSI-RS virtualization matrix, T, generated based on theweights received from base station 800, the CSI reporting for higherorder ranks may be based on the transformed channel, HT, instead ofsimply H.

In a third option, the weights for designating the virtualized CSI-RSports of 2D-MIMO active antenna array 800-AAA may be UE-specific weightsdetermined by UE 801 from rank 1 PMI feedback. For example, a rank 1 PMImay be given by, W=T₁T₂, where T₁ is a tall matrix of a UE-specificwideband precoding matrix mapping small number of antenna ports to largenumber of antenna elements, and T₂ is the subband precoding matrix for aless dimension antenna ports. In such aspects, the CSI reporting forhigh order rank may be based on, HT₁, instead of simply H.

Currently, for aperiodic CSI reporting, PUSCH mode 3-2 provides for theUE to report both subband CQI and subband PMI. However, due to theincreased feedback overhead caused by the large number of antenna ports,this mechanism is difficult to support. One solution according toadditional aspects of the present disclosure is to allow UE-selectedsubband feedback.

FIG. 9 is a block diagram illustrating example blocks executed toimplement one aspect of the present disclosure. The blocks and featuresillustrated in FIG. 9 may also be described with respect to the hardwareand components illustrated in FIG. 8. The blocks and featuresillustrated in FIG. 9 may also be described with respect to the carrier1000 illustrated in FIG. 10A. FIG. 10A is a block diagram illustratingcarrier 1000 reflecting CSI feedback operations by an UE for FD-MIMOconfigured according to one aspect of the present disclosure. At block900, a UE, such as UE 801, receives an indication from a serving basestation, such as base station 800, to report subband CQI/PMI feedbackfor a plurality of subbands (e.g., subbands 0-MN-1) over a carrier bandwidth, such as carrier bandwidth 1001.

At block 901, UE 801 selects a subset of the plurality of subbands forreporting subband CQI/PMI feedback. For example, assuming a bandwidthpart (BP) is frequency-consecutive and consists of N consecutivesubbands, for UE-selected subband feedback, a single subband out of theN consecutive subbands of a bandwidth part may be selected for CSIreporting along with an L-bit label that designates the subband locationwithin the particular bandwidth part. There are a total of M bandwidthparts for a serving cell carrier bandwidth, such as carrier bandwidth1001.

At block 902, UE 801 transmits the subband CQI/PMI feedback for each ofthe subbands in the subset. Thus, as illustrated in FIG. 8, a maximum ofM subband CQI/PMIs are reported together with M subband L-bit locationindicators. For example, instead of providing subband CQI/PMI feedbackfor each of subbands 0-MN-1, UE 801 selects, within bandwidth part (BP)0, subband 0 as the best subband for BP 0, subband MN-1 as the bestsubband for BP M-1, and any other of the indicated best subbands for thebandwidth parts between BP 0 and BP M-1. In addition to this subbandCQI/PMI feedback, UE 801 will transmit M L-bit location indicators thatindicate the location of subband 0 within BP 0, the location of subbandMN-1 within BP M-1, and the like.

It should be noted that the “best” subbands for selection by UE 801 maybe determined “best” based on the subband within the bandwidth part thatwould have the most favorable conditions for downlink transmissions frombase station 800. In additional aspects, the determination of “best”subband may be based on additional or separate criteria as well (e.g.,highest received signal strength, lowest interference, largest spectrumefficiency, and the like).

At block 903, UE 801 transmits a wideband CQI/PMI feedback determinedover the carrier bandwidth, such as carrier bandwidth 1001. In order toprovide a reference for the non-selected subbands, UE 801 may alsoreport a wideband CQI/PMI determined across the whole cell carrierbandwidth. Base station 800 may then use both the individually selectedsubband CQI/PMI feedback and the wideband CQI/PMI feedback for theentire carrier bandwidth 100 as a reference for the non-selectedsubbands in each bandwidth part.

FIG. 10B is a block diagram illustrating a carrier 1002 having subbandselection by a UE configured according to one aspect of the presentdisclosure. As an alternative solution for the distributed UE-selectedsubband feedback illustrated in FIG. 10A, UE 80 may to select wholebandwidth parts for subband CSI reporting. For example, with regard tocarrier 1000, UE 801 may select each of the subbands, subband 0-subbandN-1 for subband CSI feedback. UE 801 would generate the subband CQI/PMIfeedback for each of subband 0-subband N-1 and report this feedbackalong with a label index of bandwidth part, BP0.

FIG. 11 is a block diagram illustrating example blocks executed toimplement one aspect of the present disclosure. The blocks and featuresillustrated in FIG. 11 may also be described with respect to UE 1200illustrated in FIG. 12. FIG. 12 is a block diagram illustrating a UEconfigured according to one aspect of the present disclosure. At block1100, a UE, such as UE 1200 may receive an indication to reportaperiodic CSI feedback for a plurality of CSI-RS ports in a FD-MIMOdownlink transmission, wherein the indication includes a feedbackparameter code. For example, UE 1200 receives a control message from abase station (not shown) through antennas 352 a-r and demodulatedthrough demodulator/modulators 354 a-r, which includes one or more bitsof a feedback parameter code.

At block 1101, UE 1200 looks up a list of feedback parameters associatedwith the feedback parameter code. In operation, UE 1200, under controlof controller/processor 380 identifies the feedback parameter code inthe control signal and looks to feedback parameter table 1202 stored inmemory 382. Using the feedback parameter code, UE 1200 may identify thelist of feedback parameters associated with the code.

At block 1102, UE 1200 generates a CSI feedback report using each of thelist of feedback parameters. Using the feedback parameters identified infeedback parameter table using the feedback parameter code, UE 1200,under control of controller/processor 380 executes CSI processing 1201using the specific feedback parameters identified by the code. UE 1200may then transmit the resulting aperiodic CSI report to the requestingbase station.

Thus, the additional aspects of the present disclosure described withrespect to FIGS. 11 and 12 provide for on-demand CSI feedback. It mayalso be preferable to have on-demand CSI feedback with such specificfeedback parameters, such as CSI reporting mode, vertical vs. horizontalPMI, or both, and the like, as part of an aperiodic CSI triggering formore flexible aperiodic CSI reporting. In order to reduce L1 signalingfor flexible CSI reporting, multiple parameter sets may be predefined,as indicated, with different values for dynamic aperiodic CSI reportingparameters. A few bits added to the DCI uplink grant for the feedbackparameter code may be used to indicate which parameter set is used forflexible aperiodic CSI reporting. A UE may then use the parameter setsindicated by higher layer signalling for determining the aperiodic CSIreporting parameters. The on-demand CSI feedback via L1 signaling canprovide the best tradeoff between performance and feedback overhead.

The following parameters for determining aperiodic CSI reporting and CSImeasurement antenna port may be included in the parameter set:

Aperiodic CQI reporting mode, e.g. subband or wideband CQI/PMI

CQI/PMI beta offset for single and multiple codewords

codebookSubsetRestriction

Bandwidth part indicator

PMI/RI reporting indicator

SU/MU CSI indication

Vertical vs. Horizontal or both CSI indication

The configured parameter sets can apply to all the CSI processes or aparticular CSI process based on higher layer configuration.

Currently, the network can trigger the aperiodic CSI-only transmissionon PUSCH if there is no transport block for the UL-SCH. The followingcriteria are currently known for determining whether there is onlyaperiodic CSI feedback for the current PUSCH reporting mode. If DCIformat 0 is used or, if DCI format 4 is used and only 1 transport blockis enabled and, for the enabled transport block and the number oftransmission layers is 1, and if the “CSI request” bit field is 1 bitand the bit is set to trigger an aperiodic report and N_PRB<=4 (e.g.,for non-CA), Or the “CSI request” bit field is 2 bits and is triggeringan aperiodic CSI report for more than one serving cell according toTable 7.2.1-1A and N_PRB<=20 (for CA). Now with 2D feedback for FD-MIMOoperations, the above condition of N_PRB<=4 or N_PRB<=20 is modified toN_PRB<=8 or 40, respectively, due to double payload size of H- andV-PMIs.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

The functional blocks and modules in FIGS. 7, 9, and 11 may compriseprocessors, electronics devices, hardware devices, electronicscomponents, logical circuits, memories, software codes, firmware codes,etc., or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and process steps described inconnection with the disclosure herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or process described in connection with thedisclosure herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. A computer-readable storage medium may be anyavailable media that can be accessed by a general purpose or specialpurpose computer. By way of example, and not limitation, suchcomputer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM orother optical disk storage, magnetic disk storage or other magneticstorage devices, or any other medium that can be used to carry or storedesired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, non-transitory connections may properly be includedwithin the definition of computer-readable medium. For example, if theinstructions are transmitted from a website, server, or other remotesource using a coaxial cable, fiber optic cable, twisted pair, ordigital subscriber line (DSL), then the coaxial cable, fiber opticcable, twisted pair, or DSL are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blue-raydisc where disks usually reproduce data magnetically, while discsreproduce data optically with lasers. Combinations of the above shouldalso be included within the scope of computer-readable media.

As used herein, including in the claims, the term “and/or,” when used ina list of two or more items, means that any one of the listed items canbe employed by itself, or any combination of two or more of the listeditems can be employed. For example, if a composition is described ascontaining components A, B, and/or C, the composition can contain Aalone; B alone; C alone; A and B in combination; A and C in combination;B and C in combination; or A, B, and C in combination. Also, as usedherein, including in the claims, “or” as used in a list of itemsprefaced by “at least one of” indicates a disjunctive list such that,for example, a list of “at least one of A, B, or C” means A or B or C orAB or AC or BC or ABC (i.e., A and B and C).

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method of wireless communication, comprising:receiving, at a user equipment (UE), an indication to report channelquality and precoding feedback for a plurality of channel stateinformation reference signal (CSI-RS) ports in a full dimensionalmultiple input, multiple output (FD-MIMO) downlink transmission;transmitting, by the UE, a first lower-order rank channel quality andprecoding feedback for each of the plurality of CSI-RS ports, whereinthe first channel quality and precoding feedback includes a low rankindicator; selecting, by the UE, a second subset of CSI-RS ports of theplurality of CSI-RS ports for a second higher-order rank channel qualityand precoding feedback, wherein the selecting the second subset ofCSI-RS ports, which represent a fewer number of ports than a totalnumber of the plurality of CSI-RS ports, is based on a rank indicatorassociated with the second higher-order rank channel quality andprecoding feedback; and transmitting, by the UE, the second higher-orderrank channel quality and precoding feedback for each of the secondsubset of CSI-RS ports.
 2. The method of claim 1, wherein the selectingincludes: receiving, at the UE, a bitmap indicator signal from a servingbase station, wherein the bitmap indicator signal identifies the CSI-RSports in the plurality of CSI-RS ports for the second higher-order rankchannel quality and precoding feedback; and selecting the second subsetof CSI-RS ports using the bitmap indictor.
 3. The method of claim 1,wherein the selecting includes: receiving, at the UE, weightinginformation from a serving base station; and mapping, by the UE, theplurality of CSI-RS ports into the second subset of CSI-RS ports usingthe weighting information.
 4. The method of claim 1, further including:determining, by the UE, weighting information based on the firstlower-order rank channel quality and precoding feedback; and mapping, bythe UE, the plurality of CSI-RS ports into the second subset of CSI-RSports using the weighting information.
 5. An apparatus configured forwireless communication, comprising: means for receiving, at a userequipment (UE), an indication to report channel quality and precodingfeedback for a plurality of channel state information reference signal(CSI-RS) ports in a full dimensional multiple input, multiple output(FD-MIMO) downlink transmission; means for transmitting, by the UE, afirst lower-order rank channel quality and precoding feedback for eachof the plurality of CSI-RS ports, wherein the first lower-order rankchannel quality and precoding feedback includes a low rank indicator;and means for selecting, by the UE, a second subset of CSI-RS ports ofthe plurality of CSI-RS ports for a higher-order rank second channelquality and precoding feedback, wherein the selecting the second set ofCSI-RS ports, which represent a fewer number of ports than a totalnumber of the plurality of CSI-RS ports, is based on a rank indicatorassociated with the second higher-order rank channel quality andprecoding feedback; means for transmitting, by the UE, a secondhigher-order rank channel quality and precoding feedback for each of asecond subset of CSI-RS ports.
 6. The apparatus of claim 5, wherein themeans for selecting includes: means for receiving, at the UE, a bitmapindicator signal from a serving base station, wherein the bitmapindicator signal identifies the CSI-RS ports in the plurality of CSI-RSports for the second higher-order rank channel quality and precodingfeedback; and means for selecting the second subset of CSI-RS portsusing the bitmap indictor.
 7. The apparatus of claim 5, wherein themeans for selecting includes: means for receiving, at the UE, weightinginformation from a serving base station; and means for mapping, by theUE, the plurality of CSI-RS ports into the second subset of CSI-RS portsusing the weighting information.
 8. The apparatus of claim 5, furtherincluding: means for determining, by the UE, weighting information basedon the first lower-order rank channel quality and precoding feedback;and means for mapping, by the UE, the plurality of CSI-RS ports into thesecond subset of CSI-RS ports using the weighting information.